The present invention relates to a series compensating electric power transmission system that corresponds to an electric power transmission system in which electric power is transmitted bidirectionally between a first DC (direct current) voltage source and a second DC (direct current) voltage source. The series compensating electric power transmission system has a DC-DC converter that outputs a compensation voltage that corresponds to a voltage difference between the DC voltage sources.
Various series compensating electric power transmission systems are disclosed in Japanese Patent Publication Number 2012-44801 (pages 3-15 and FIGS. 2-14; various series compensating electric power transmission systems are disclosed as background and various series compensating electric power transmission systems are disclosed as embodiments).
As shown in FIG. 17, the series compensating electric power transmission system 101 mentioned above is basically configured with a primary side DC voltage source 11 (its output voltage (a DC voltage) corresponds to V1) as a first DC voltage source, a secondary side DC voltage source 12 (its output voltage (a DC voltage) corresponds to V2) as a second DC voltage source, and a DC-DC converter (a bidirectional DC-DC converter) 102 that outputs a compensation voltage Vconv, which corresponds to a voltage difference between the output voltages V1 and V2, by connecting output terminals between a positive voltage of the primary side DC voltage source 11 and a positive voltage of the secondary side DC voltage source 12 in series.
In the series compensating electric power transmission system 101, a negative output terminal of the primary side DC voltage source 11 and a negative output terminal of the secondary side DC voltage source are connected. Further, in regards to a pair of first DC input and output terminals P1 and P2 of the DC-DC converter 102, the first DC input and output terminal P1 that corresponds to a positive side is connected to a positive output terminal of the primary side DC voltage source 11, and the first DC input and output terminal P2 that corresponds to a negative side is connected to the negative output terminal of the primary side DC voltage source 11. Further, in regards to a pair of second DC input and output terminals Q1 and Q2 of the DC-DC converter 102, the second DC input and output terminal Q1 is connected to a positive output terminal of the secondary side DC voltage source 12, and the second DC input and output terminal Q2 is connected to the positive output terminal of the primary side DC voltage source 11.
In the series compensating electric power transmission system 101 that is disclosed as the embodiment among the various series compensating electric power transmission systems 101 disclosed in Japanese Patent Publication Number 2012-44801, the DC-DC converter 102 is shown in FIG. 18. As shown in FIG. 18, the DC-DC converter 102 is configured with a primary side power conversion unit 111, a high frequency transformer (an isolation transformer) 112 and a secondary side power conversion unit 113. Specifically, the primary side power conversion unit 111 has the pair of first DC input and output terminals P1 and P2 that are connected to the positive output terminal and the negative output terminal of the primary side DC voltage source 11, and at the same time, the primary side power conversion unit 111 converts the output voltage V1 into an AC (alternating current) voltage and outputs the AC voltage. The high frequency transformer (the isolation transformer) 112 receives the AC voltage as an input that is output from the primary side power conversion unit 111, converts it into another AC voltage and outputs the other AC voltage. The secondary side power conversion unit 113 outputs the compensation voltage Vconv based on the other AC voltage that is output from the high frequency transformer 112. As a result, the DC-DC converter 102 is configured as an insulated type DC-DC converter. In the embodiment of Japanese Patent Publication Number 2012-44801, it is disclosed that a full bridge type power converter or a half bridge type power converter can be used as the primary side power conversion unit 111 and the secondary side power conversion unit 113.
However, as schematically shown in FIG. 18, in the configuration of the DC-DC converter 102 that is disclosed in the embodiment of Japanese Patent Publication Number 2012-44801, two diodes are connected to two switches in parallel, respectively, between the pair of the second DC input and output terminals Q1 and Q2 (that is, between both positive output terminals of the output voltages V1 and V2). The two diodes are connected in series so as to be provided in a forward direction. In other words, current flow directions of the two diodes are the same to each other. Specifically, the two switches (in FIG. 18, as an example, field effect transistors) configure the secondary side power conversion unit 113 that corresponds to the bridge type power converter as explained above. Therefore, the DC-DC converter 102 can be operated only under a condition in which the output voltage V1 is lower than an output voltage V2, and output the voltage difference (V2−V1>0) between the output voltages V2 and V1 as the compensation voltage Vconv during a period of the above conditional operation. That is, in the series compensating electric power transmission system 101 that is disclosed in the embodiment of Japanese Patent Publication Number 2012-44801, the DC-DC converter 102 is operated only under the condition that satisfies an expression, V2>V1, so that the DC-DC converter 102 performs an operation of supplying the electric power to the secondary side DC voltage source 12 from the primary side DC voltage source 11 (a power-running operation) and an operation of supplying the electric power to the primary side DC voltage source 11 from the secondary side DC voltage source 12 (a regenerative operation).
On the other hand, in the series compensating electric power transmission system 101 (a series compensating electric power transmission system that is disclosed as a non-patent literature 3 in Japanese Patent Publication Number 2012-44801) that is disclosed in the background among the various series compensating electric power transmission systems 101 disclosed in Japanese Patent Publication Number 2012-44801, though a drawing is omitted, an insulated type DC-DC converter that is configured by combining a flyback converter (a converter body) with a H-Bridge is used as the DC-DC converter 102. Further, a DC-DC converter that is configured with a transformer-less polarity inversion chopper circuit (a circuit that has an H-Bridge circuit) is used as the DC-DC converter 102.
In the DC-DC converter 102 having the configuration explained above, because the H-bridge circuit performs a four-quadrant chopper operation in a time near switching between a step-up operation and a step-down operation according to a polarity of the voltage difference between the output voltages V1 and V2, it is possible to perform an operation of supplying the electric power to the secondary side DC voltage source 12 from the primary side DC voltage source 11 (a power-running operation in the first quadrant) and an operation of supplying electric power to the primary side DC voltage source 11 from the secondary side DC voltage source 12 (a regenerative operation in the fourth quadrant) under the condition in which the output voltage V1 is lower than the output voltage V2. At the same time, though there is no description regarding the operation including control methods in the non-patent literature, it is also possible to perform an operation of supplying the electric power to the secondary side DC voltage source 12 from the primary side DC voltage source 11 (a power-running operation in the second quadrant), and an operation of supplying electric power to the primary side DC voltage source 11 from the secondary side DC voltage source 12 (a regenerative operation in the third quadrant) under the condition in which the output voltage V1 is higher than the output voltage V2, according the disclosed configuration.
Among the insulated type DC-DC converters disclosed in Japanese Patent Publication Number 2012-44801, the DC-DC converter 102 of the series compensating electric power transmission system 101 that is disclosed in the background explained above can operate in the four quadrants by control. However, because a converter body has a two-stage configuration of the flyback converter and the H-Bridge, a configuration and control of the DC-DC converter are complicated. Therefore, there are problems to be solved, such as a decrease in conversion efficiency and an increasing in size of the converter body.
On the other hand, in regards to the DC-DC converter 102 explained above of the series compensating electric power transmission system 101 that is disclosed in the embodiment of Japanese Patent Publication Number 2012-44801, because just a converter body is needed, the problems in the DC-DC converter 102 that is disclosed in the background of Japanese Patent Publication Number 2012-44801 do not exist. However, as explained above, there is the problem that the electric power can be transmitted only under the condition in which the output voltage V1 is lower than the output voltage V2.